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Optical frequency measurements of the intercombination line (6s 2 ) 1 S0 −(6s6p) 3 P1 in the isotopes of ytterbium are carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic beam. A dispersive signal is generated to which a master laser is locked, while frequency counting of an auxiliary beat signal is performed via a frequency comb referenced to a hydrogen maser. The relative separations between the lines are used to evaluate the 3 P1-level magnetic dipole and electric quadrupole constants for the fermionic isotopes. The center of gravity for the 3 P1 levels in 171 Yb and 173 Yb are also evaluated, where we find significant disagreement with previously reported values. These hyperfine constants provide a valuable litmus test for atomic many-body computations in ytterbium.

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We demonstrate a means of detecting weak optical transitions in cold atoms that undergo cyclic (preparation, probing, and detection) routines with improved sensitivity. The gain in sensitivity is made by probing atoms on alternate cycles of a repeated experimental sequence, leading to regular modulation of the ground state atom population when at the resonance frequency. The atomic transition is identified by conducting a fast Fourier transform via an algorithm or instrument. We find an enhancement of detection sensitivity compared to more conventional scanning methods of ∼20 for the same sampling time, and can detect contemporary clock lines with fewer than 103 atoms in a magneto-optical trap. We apply the method to the (6s2) 1S0−(6s6p) 3P0 clock transition in 171Yb and 173Yb. In addition, the ac-Stark shift of this line in 171Yb is measured to be 0.19(3) kHz W−1 m2 at 556 nm.